In heterogeneous networks with a mix of different cell sizes, typically different downlink power levels are used, which render different cell sizes. For example between macro sites and micro sites there can be 10-20 dB difference. There are a number of different nodes that are expected to be deployed with different downlink power levels such as e.g. macro base stations, micro base stations, pico base stations, femto base stations, relays and repeaters.
A macro base station may be referred to as a conventional base station that use dedicated backhaul and is open to public access. Typical transmit power may be e.g. ˜43 dBm; antenna gain ˜12-15 dBi.
A pica base station may be a low power base stations that use dedicated backhaul connections and is open to public access. Typical transmit power may range from ˜23 dBm-30 dBm, 0-5 dBi antenna gain.
A femto base station may be a consumer-deployable base station that utilize consumer's broadband connection as backhaul; femto base stations may have restricted association. Typical transmit power may be less than 23 dBm.
Relays—base stations possibly use the same spectrum as backhaul and access. Typical transmit power may be of the same magnitude as pico base stations.
In the present context, the expression downlink is used for the transmission path from a base station such as e.g. an eNodeB, to the user equipment unit (UE). The expression uplink is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
Home eNodeB (HeNB or femto cells) are discussed in 3GPP for Long Term Evolution (LTE). A specific X2 interface is expected to be defined for this type of nodes, i.e. a communication interface between the base stations/eNodeBs/HeNB. X2 is the designated name of a standardized interface between two eNodeBs in E-UTRAN. The X2 can be seen as a logical connection between two eNodeBs over which user data and signaling messages are exchanged.
For uplink the power capability in the user equipment is independent of eNodeB type. In LTE downlink power is typically constant and uplink power is controlled with standardized open-loop power control. The target uplink received power is controlled with a parameter P0, the uplink received power target, which also may be referred to as the target received power. The target uplink received power P0 is the power level the base station desires the uplink signals from a user equipment to be received at.
LTE is designed for a 1-reuse resulting in a wideband Signal-to-Interference-Ratio (SIR) of 0 dB or less at the cell edges in a fully loaded network.
A cell selection and/or handover offset can be applied to increase the micro-cell coverage picking up all user equipments creating high interference in the smaller cell. One similar method is to base the cell selection on path loss instead of signal strength which is the same as an offset equal to the downlink power difference.
If the same target received power level is used in uplink, while downlink power differs between cells, there is an unbalance in received power levels between uplink and downlink when connected to a single site. It is further so that a user equipment connected to the macro base station, but close to the micro base station will generate an interference far exceeding the power of the desired signal in the micro base station. This results in a very low SIR. See further FIG. 1A and 1B. In FIG. 1A is downlink received power displayed, in FIG. 1B is uplink received power displayed.
Equal received power target (−90 dBm) in macro and micro base stations. Note that the SINR in the micro base station (MBS) is very low due to the interference from the user equipments (MS) connected to the macro base station (BS).
This may be mitigated by increasing the received power target in the micro base stations. This however leads to a very high interference in the macro base station, see FIG. 2A and 2B, wherein downlink and uplink received power, respectively, is illustrated. An increased received power target in the micro base station (−40 dBm) leads to very high interference in the macro base station.
One way of increasing the received power in the micro base stations is to artificially increase the noise level in these base stations, which may be referred to as desensitization. However, if the noise level is increased in the smaller cells there is a waste of good SIR. With a cell selection handover offset the downlink bitrate in the micro cell will be degraded.